1. Field of the Invention
The present invention relates generally to the field of wavelength division multiplexed optical communication systems and, more particularly, to an add-drop multiplexer for transferring selected optical channels between transmission paths within a wavelength division multiplexed optical communication system
2. Technical Background
Optical communication systems transmit optical signals over a wide spectrum of optical wavelengths. Such as for example, 1530 nm to 1565 nm. To more efficiently use the optical communication systems, the transmission spectrum is further divided into sub-wavebands or channels for the simultaneous transmission of multiple data streams. This simultaneous transmission of these data streams on different channels is referred to as wavelength division multiplexing. Optical communication systems that utilize wavelength division multiplexing require add/drop multiplexers in order to introduce and remove individual channels from the optical communication system. Preferably, these add/drop multiplexers should have narrow and accurate filtering characteristics. Narrow and more accurate filtering characteristics of the add/drop multiplexers enable closer channel spacing, which in turn increases the number of channels that an optical communication system can transmit.
Fiber Bragg gratings may be used to provide the filtering function in such an add/drop multiplexer. Typically, however, fiber Bragg gratings have a fixed filter wavelength and thus may only be used to add or drop a single, predetermined channel. Give cost and upgradeablity concerns, the deployment of flexible networks incorporating reconfigurable optical add/drop multiplexers is desirable. Reconfigurable add drop multiplexers are required in order to dynamically select the channel or channels to be added or dropped. The optical filters used in reconfigurable add/drop multiplexers need to have the correct bandwidth and be tunable so that the waveband reflected by the filter corresponds to channels to be added or dropped.
Two ways in which the center wavelength of a fiber Bragg grating may be tuned or changed are by utilizing the elastooptic or the thermooptic properties of the grating. The elastooptic properties of the grating may be advantageously used by straining to the grating. Typically, the shift in center wavelength is Ipm per microstrain. Additionally, the thermooptic properties of the grating may be exercised by changing the temperature of grating. Heating or cooling the grating shifts the center wavelength of the grating. For example, at a wavelength of 1550 nm the center wavelength of the grating shifts about 10 pm per degree Kelvin.
For a single fiber Bragg grating to be useful in a reconfigurable optical add/drop multiplexer, in must be capable of having its center wavelength shifted by several nanometers. Piezoelectric strain transducers have demonstrated the capability to shift the Bragg wavelength of a grating at the rate of about 2.4 nm per 100V.
One aspect of the invention is an optical add/drop multiplexer. The optical add/drop multiplexer includes a first optical circulator, the first circulator includes a first port, a second port, and a third port. The optical add/drop multiplexer also includes a first tunable grating coupled to the first port and a second tunable grating coupled to the first grating. The optical add/drop multiplexer further includes a second optical circulator, the second optical circulator includes a fourth port, coupled to the second grating; a fifth port, and a sixth port. Wherein the first tunable grating is a fiber Bragg grating and wherein the second tunable grating is a fiber Bragg grating.
In another aspect, the present invention includes an optical add/drop multiplexer. The optical add/drop multiplexer includes an optical circulator. The optical circulator including a first optical port, a second optical port and a third optical port. Wherein optical signals received by the first port are directed to the second port. Wherein optical signals received by the second port are directed to the third port. Wherein optical signals received by the third port are directed to the first port. The optical drop/add multiplexer further includes a first tunable grating coupled to the second optical port and a first tuning member coupled to the first tunable grating. Wherein the tuning member is selectively positionable in at least three positions. Wherein each of the at least three positions corresponds to a different center wavelength for the first tunable grating.
In another aspect, the present invention includes a method for removing at least a portion of an optical signal. The method includes the step of providing a first optical circulator, the first optical circulator having a first port, a second port, and a third port. The method also includes the steps of providing a first tunable grating and coupling the first tunable grating to the second port. The method further includes the steps of providing a second tunable grating and coupling the second tunable grating to the first tunable grating. The method further includes the step of providing a second optical circulator, the second optical circulator having a fourth port, a fifth port, and a sixth port. The method further includes the step of coupling the second tunable grating to the fourth port. The method further includes the steps of tuning the first tunable filter to a first predetermined center wavelength and linewidth and tuning the second tunable filter to a second predetermined center wavelength and linewidth. The method further includes the step of introducing an optical signal into the first port; wherein at least a portion of the optical signal is reflected by either the first tunable grating or the second tunable grating.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely illustrative of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.